The EP application 07 019 555.7 describes a sealing device for pipes, which contains correspondingly flexible soft metals or soft metal alloys such as tin, lead, indium, gold, copper, aluminum, in particular metal alloys with a Mohs hardness between 1 and 3 as sealing compound, and methods for the manufacture and use thereof, which can be used in pipes or bodies to be sealed of similar type and construction, in particular instead of seals of elastomeric and/or polymeric materials.
In particular, said application relates to a novel and highly effective arrangement of the sealing elements which provide for a substantial improvement of the tightness of sealing systems in particular with respect to volatile media such as hydrogen and helium.
The advantage of such seal consists in a substantial reduction of the diffusion of extraneous substances, i.e., gases with an extremely low molecular weight, through the sealing region, and as compared to plastic seals in achieving very much better tightness values by using flexible, soft metals or metal alloys as sealing material.
In the ideal case, a sufficient tightness between two surfaces could also be achieved without the aid of sealing materials. For this purpose, however, the surfaces to be sealed should have a smooth surface not achievable in practice, or the force which urges the surfaces to be sealed against each other would have to be so great that all cavities still present between the sealing surfaces are leveled out by material flow to such an extent that the molecules of the medium to be sealed no longer can pass through the resulting boundary zones between the sealing surfaces, because their distance from each other generally is smaller than the effective molecule diameter of the gas. Experiments have shown that at such a high pressure, the boundary surfaces are deformed non-uniformly, so that they can no longer be sealed after demounting the flanges.
It is state of the art to insert a soft sealing material between the surfaces to be sealed, which upon compression of the assembly can easily and flexibly be pressed onto the sealing surfaces and into the existing cavities between the surfaces to be sealed.
A standard state of the art product are the well-known O-rings, which generally can be made of elastomer and polymer plastics, but also of soft metal. Up to now, it has been regarded as essential that the sealing material has a lower Mohs hardness than the flanges.
Understandably, the sealing of connections of pipes with flanges under high pressure or high vacuum creates particular difficulties when the medium to be sealed is hydrogen or helium, since both elements have the smallest possible molecule diameter of all volatile substances and thus also can escape through smallest leaks in the case of pressure differences. In the case of hydrogen, the known phenomenon of metal penetration should also be considered in dimensioning the sealing systems.
The described problems have long since been studied intensively and a wide variety of suggestions have been made to optimize the sealing behavior especially of flange connections of pipes.
Most of the developments of high-vacuum or high-pressure technology are based on a form of flange connection for high-vacuum systems with the above-described O-rings which is known for more than 40 years. However, the sealing properties of said O-rings with respect to e.g., helium are worse than those of metal seals at least by a factor of 10−6.
In the U.S. Pat. No. 3,208,758 copper is used as soft metal in the form of flat sealing disks, which are inserted between identical flanges such that their protruding ridge member generates an increased sealing pressure upon compression of the flanges due to a resulting inclined surface (cf. U.S. Pat. No. 5,640.751, Col. 1, lines 8-18 and Swiss Patent No. 422 448). This type of sealing was improved by a different design of the soft metal such that the above-described ring seal can be replaced by seals with virtually any surface (U.S. Pat. No. 5,640.751, Col. 2, lines 34-38).
A further approach to the improvement of sealing properties is described in GB-A 2,038,972. Here as well, bevels and grooves which upon compression of the parts to be sealed are partly filled with sealing compound or penetrate into the same are intended to increase the effective pressure in the sealing surface exposed to a vertical relative movement and thus produce a higher tightness. A disadvantage of this configuration is the difficult separation of such compounds, for instance during maintenance work (cf. p. 1, lines 31-37 in conjunction with p. 1, lines 100-104).
In U.S. Pat. No. 2,760,673 an attempt is made to achieve the sealing properties by at least two O-rings with wedge-shaped cross-section (FIGS. 1, 17 and 18). For the specific increase in pressure between the O-rings, a correspondingly equipped channel (20, 20a) is required here, which unnecessarily complicates the sealing mechanism and in practice would rather lead to undesired leakages, which likewise question the advantage of such a changed sealing surface especially in the case of highly volatile gases such as helium (Col. 1, lines 56-64),
The FR Patent No 1,044.153 describes a kind of double-wailed Dewar vessel with an inner copper tank, which should be used as autoclave for high-pressure reactions e.g., in liquid oxygen. By means of wedge-shaped parts of the sealing ring (FIGS. 1, 3a) a particular pressure resistance in the case of sudden changes in temperature is achieved (p. 2, Col. 1, penultimate and last sentence) and at the same time is said to very much facilitate the opening and closing of the vessel. An improvement of the sealing quality when using soft metal sealing material is not described.
The FR Patent No. 1,506,567 describes a lens-shaped soft metal seal, in which in the case of vertical compression the pressure acting on the thicker center piece of the horizontally applied sealing disk should generate an increased transverse pressure by leveling out the central bulge to improve edge sealing (Col. 2, para. 1-3).
A further metal seal is described in DE-A 24 16 808 for vacuum purposes. For easier handling only two sealing rings (a sealing ring and a centering ring) here substantially are combined with each other such that the sealing connection can be separated again in a particularly easy way. Even in the case of a great deformation, only the sealing ring must then be replaced, and the centering ring can be used as often as desired (p. 2, para. 2 and 3).
In DAS 1 228 871 a sealing arrangement for high-vacuum flange connections is described, in which the sealing ring of soft metal fills a cavity formed of two opposed grooves, when the two flanges are compressed. In FIGS. 1 and 3 it is shown that by designing the cavity with sharp teeth and bevels of the walls a particular shape is intended. The alleged advantages of this arrangement are described in Col. 3, lines 30-47 and hardly relate to an improvement of the tightness.
The U.S. Pat. No. 3,038,731 likewise relates to soft metal seals for achieving a high vacuum in vessels or conduits made of copper, brass, stainless steel, nickel, molybdenum, tungsten, tantalum, glass and synthetic MICA silicate, whose tightness is improved by liquefying the bearing surface of the sealant under pressure. Gallium, tin, indium, bismuth and lead and the alloys thereof above all are proposed as sealant. Liquefaction is achieved by soldering or directly applying the liquid sealant (Col. 2, lines 50-62). A particular material-related thickness of the sealing layer should be ensured, which in particular in the case of alloys is adjustable in accordance with the invention (Table I). A particular shape of the seals is not important here, but the application thereof is very complicated.
In DE-A 1 425 429 a high-vacuum seal in particular for valves is described, which in particular deals with the problems of cold welding on valve surfaces of vacuum pumps. As proposed, a high-vacuum seal should be created, in which on the one hand the irregularities of the sealing surface, which are left even after the best surface treatment, should be closed without the materials of the sealing element being welded to each other. A particular shape or arrangement of the seal is not proposed here (except for a preferably conical formation of the sealing surfaces).
According to the prior art discussed above, O-ring arrangement with helium leakage rates of not more than <10−6 mbar/s have been achieved. However, as compared to the current scientific and technical requirements, this is no longer sufficient. Today, leakage rates of less than 10−10 mbar/s in part are required.
Therefore, it was the object of the invention according to EP 07 019 555.7 to manufacture sealing arrangements to be manufactured in an uncomplicated and inexpensive way with very much better leakage rates possibly even below the detection limit of commercially available helium leak detecting devices (about <10−11 mbar/s).
For gas seals, based on the prior art, one had to fall back on the known favorable properties of correspondingly suitable soft metal compositions or stable plastics such as highly fluorinated hydrocarbons.
The solution of the object was based on the surprising discovery that the sealing material used between flat sealing surfaces such as in flanges with smooth surfaces in the form of a flat ring with rectangular cross-section is distributed under pressure with respect to the distribution in the cross-section such that it radially “flows” to the outside and undergoes a permanent plastic deformation. Hence it can be concluded that materials exhibiting an elastic behavior under the pressing conditions are less suitable. Elastic materials withstand a deformation. Under extreme pressure, plastic materials however flow into existing depressions of the sealing surface and completely fill the same permanently. Under high pressure, the outside diameter of the sealing ring now grows to a relatively greater extent than the inside diameter. Thus, the volume of the sealing ring rectangular in cross-section and lying flat on a planar surface is changed under the pressure of a plunger such that a greater part of the sealing compound is shifted radially to the outside than could have been expected by a geometrically uniform distribution. This effect of the radial flow of the sealing compound to the outside has already been utilized by the teaching of the above-mentioned EP application, in that the otherwise smooth flow of the sealing material is obstructed by oppositely directed stair-like steps between the bearing surfaces such that at the perpendicular blocking surfaces formed by the stairs an unexpectedly high pressure is built up, which in turn greatly increases the effectiveness of the sealing material.
This also results in the knowledge that the substantial sealing by the measures in accordance with the invention surprisingly does not occur at the plunger surfaces, but at those surfaces which are parallel to the direction of the actual pressing pressure and perpendicular to the plunger and flange planes. Hence it follows that the sealing of the space between opposed flange planes regarded as the only important sealing according to the known prior art only is of secondary importance. In application of this knowledge, the demands as to the smoothness of the surfaces of the sealing elements surprisingly are greatly reduced, and additional surface treatment agents, such as fats, oils, silicones or similar lubricants, are not required either to achieve the desired tightness.
Tests have shown that due to the above-described configuration of sealing elements an improvement of the sealing properties, which so far has hardly been deemed to be possible, can be achieved in particular for highly volatile gases such as hydrogen or helium. As compared to usual helium leakage rates (10−6 mbar/s), this seal has provided an improvement to between 10−9 mbar/s and 10−12 mbar/s. Especially zinc, lead, indium, gold and further soft metals such as copper and aluminum are useful as sealing material. Correspondingly soft alloys of metals can also be used. The metals and alloys should have a Mohs hardness between 1 and 3, preferably between 2 and 3, it is particularly important that with a closed seal the opposed flanges mechanically contact each other in a frictional manner and thus firmly rest against each other. This ensures that the seal itself is completely kept free of forces acting from outside, which can greatly deteriorate the sealing quality.